Smectic State Research Articles

Disclination unbinding transition in quantum Hall liquid crystals

We derive the the long-wavelength elastic theory for the quantum Hall smectic state starting from the Hartree-Fock approximation. Dislocations in this state lead to an effective nematic model for $T>0$, which undergoes a disclination unbinding transition from a phase with algebraic orientational order into an isotropic phase. We obtain transition temperatures which are in qualitative agreement with recent experiments which have observed large anisotropies of the longitudinal resistivities in half-filled Landau levels, lending credence to the liquid crystal interpretation of experiments.

Nearly field-independent in-plane vortex solid-to-liquid transition in thec-axis resistivity of oxygen deficient single crystals ofYBa2Cu3O7−δ

The c-axis resistivity has been measured in oxygen deficient single crystals of YBa2Cu3O7-delta (anisotropy gamma approximate to 24-29) with the magnetic field B accurately aligned parallel to the ab plane. An almost field-independent vortex solid-to-liquid transition is observed at high fields in contrast to the usual field-dependent one, which is recovered for a small misalignment between B and ab. This observation challenges previous interpretations of the same effect in the ab resistivity as coming from a smectic vortex state or a decoupling of the superconducting planes.

Rigid‐chain metallomesogenic polymers containing vanadyl or copper(II) ions coordinated in the main chain

AbstractRigid‐chain metallomesogenic polymers containing Cu(II) or VO(II) were prepared and characterized. All the polymers were soluble and melted without decomposition. They showed a thermotropic liquid–crystal (LC) behavior, and the mesophases were invariably preserved for a long time at room temperature in a metastable condition, with respect to the semicrystalline state. The nature of the mesophases of the Cu(II)‐containing polymers was similar to that observed in analogous organic rigid‐chain polymers having long lateral alkyl chains packed as extended ribbons. The VO(II)‐containing polymers showed an LC polymorphism involving a smectic A and a nematic phase. For all the polymers in a smectic or smecticlike state at room temperature, X‐ray diffraction data suggested short mean distances among the metal ions arranged in layers. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2342–2349, 2001

Shape and position of middle-angle scattering peaks from smectic phases of polymers

A method for the quantitative analysis of isotropic middle-angle X-ray scattering (MAXS) from segmented polymers, which are able to form liquid-crystalline (LC) phases, is proposed and applied to scattering curves from poly(ester imide) samples. Oriented material in the LC state shows MAXS fiber patterns with a meridional layer-line reflection, which sometimes is split, creating a four-point pattern. Comparing scattering data from oriented material with data from isotropic material, we can explain the asymmetry of the isotropic MAXS reflection by limited lateral correlation. Thus, for the LC state of segmented polymers, the notion of stacks built from alternating planar layers of smectic mesogens and spacers units is not generally applicable. Only in the limiting case of infinite flatness does the line profile become symmetric. A simple analytical expression for the shape of the line profile of MAXS reflections is deduced. The equation fitted to the MAXS peak of isotropic LC polymer samples in the smectic state results not only in the determination of a corrected long period but also in two additional parameters, the correlation height of the smectic stack and the flatness of its layers. Compared to established methods for line-profile analysis in small-angle scattering, the proposed method models the nanostructure with classical notions of a two-phase system in one direction only, whereas the lateral contribution to the scattering intensity is characterized by a short-range loss of correlation instead of a classical description by domain-phase boundaries. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1749–1755, 2001

Oscillatory melting temperature of the vortex smectic phase in layered superconductors

We report on transport measurements of YBa 2Cu 3O (7-delta) single crystals with different oxygen contents in the geometry B, J ||ab (J perpendicularB). Our data show that the vortices become confined between the Cu-O planes below a well-defined temperature at which the effective size 2xi of the vortex core is approximately equal to the period of the Cu-O layers. This confinement strongly increases the vortex liquid freezing temperature. A new melting line is found separating a vortex liquid and a smectic phase, which shows an oscillatory field dependence reflecting differences between commensurate and incommensurate smectic states.

Quantum theory of the smectic metal state in stripe phases

We present a theory of the electron smectic fixed point of the stripe phases of doped layered Mott insulators. We show that in the presence of a spin gap three phases generally arise: (a) a smectic superconductor, (b) an insulating stripe crystal, and (c) a smectic metal. The latter phase is a stable two-dimensional anisotropic non-Fermi liquid. In the absence of a spin gap there is also a more conventional Fermi-liquid-like phase. The smectic superconductor and smectic metal phases (or glassy versions thereof) may have already been seen in Nd-doped La2-xSrxCuO4.

Bistable Memory Switching of (Liquid Crystalline Copolymer/ Low Molecular Weight Liquid Crystal) Composite Systems

Bistable electro-optical switching characteristics have been investigated for composite systems in a smectic phase induced by mixing a side chain type nematic liquid crystalline polymer (nematic LCP) or its copolymer with weak polar methoxy terminal groups in the side chains and low molecular weight nematic liquid crystals (nematic LCs) with strong polar cyano end groups. The liquid crystalline copolymer (LCcoP) with weak polar methoxy terminal groups in the side chains attaching to a polysiloxane main chain was used in order to improve the response speed of the bistable light switching for the binary composite system. Although the LCcoP with 52.5 mol% substituted mesogenic side chains did not exhibit any mesophase characteristics at any temperature, this LCcoP induced a smectic state by mixing nematic LCs. A reversible and bistable electro-optical switching with a short response time (∽100 ms) was realized for the induced smectic binary composite system upon the application of an appropriate electric field at room temperature.

Melting of moving vortex lattices in systems with periodic pinning

We study numerically the effects of temperature on moving vortex lattices interacting with periodic pinning arrays. For low temperatures the vortex lattice flows in channels, forming a hexatic structure with long range transverse and longitudinal ordering. At higher temperatures, a transition to a smectic state occurs where vortices wander between channels and longitudinal order is lost while transverse order remains. At the highest temperatures the vortex lattice melts into an isotropic liquid.

Quantum theory of quantum Hall smectics

We propose a quantum stripe (smectic) coupled-Luttinger-liquid model for the anisotropic states which occur in two-dimensional electron systems with high-index partial Landau level filling, $\nu^{*} = \nu - \lbrack\nu\rbrack$. Perturbative renormalization group calculations establish that interaction terms neglected in this model are relevant - probably driving the system into an anisotropic Wigner crystal---but for $0.4 \lesssim \nu^{*} \lesssim 0.6$ only below temperatures which are outside of the experimentally accessible range. We argue that the Hall conductance of the ground state flows toward $\lbrack\nu\rbrack e^{2}/h$ and $(\lbrack\nu\rbrack + 1) e^{2}/h$ respectively, on the low and high filling factor sides of this range, consistent with recent observations. A semiclassical theory of smectic state transport properties, which incorporates Luttinger liquid effects in the evaluation of scattering amplitudes, accounts for the magnitude of the dissipative resistivities at $\nu^{*}=1/2$, for their $\nu^{*}$-dependence, and for the observation of non-linearities of opposite sign in easy and hard direction resistivities.

Order To Disorder Transition at The Polymer-Air Interface for Hydrophobic Octadecyl Side Chain Copolymers

ABSTRACTMolecular orientation of octadecyl alkyl side chains at the poly (vinyl octadecyl carbamates-co-vinyl acetate) polymer-air interface has been studied using surface sensitive sum frequency generation (SFG) spectroscopy. At 280C, below the side chain crystalline transition temperature, the SFG spectra show strong methyl vibrations indicating ordered alkyl side chains at the polymer-air interface. In the liquid state (980C), the SFG spectra show higher contributions from the methylene vibrations indicating higher gauche defects at the interface. This surface order to disorder transition is gradual and spans over a broad temperature range (50-900C), where the bulk is in the smectic liquid crystalline state.

A smectic liquid crystal containing three aromatic units which can be deposited by the LB method

We have synthesised a fumarate ester containing three aromatic rings and studied its properties in the bulk by DSC and other techniques. We have also deposited films of this compound by the LB method and by thermal evaporation in vacuo and characterised multilayers of it by low angle X-ray diffraction. DSC results show that the compound has a crystal to liquid crystal transition at 80°C and a liquid crystal to liquid transition at about 150°C. We have been able to photo-polymerise it while it is in the smectic state. Newly deposited multilayers exhibit clear X-ray diffractograms. However, if the multilayers are annealed at a temperature within the liquid crystalline region, the diffractograms become more clearly defined and the Bragg peaks increase in amplitude indicating an improvement in the coherence length in the direction normal to the plane. We have succeeded in depositing alternate layers of this compound and another compound having liquid crystal properties which we have previously studied. These have been exposed to radiation having a wavelength of 1.064 μm in an attempt to generate the second harmonic but without success. In view of the structure of the compound which has a series of ester bonds, each having an electric dipole moment, we assume that the conformation of the compound is such that these dipoles do not reinforce each others influence as otherwise we would have expected to have observed at least a weak second harmonic signal. Cast films of the compound heavily diluted with an inactive polymer and poled do show some SHG properties.

Horizontal chevron domain formation and smectic layer reorientation in SmC* liquid crystals stabilized by polymer networks

The influence of a polymer network, stabilizing an initial texture of horizontal chevron geometry, on the in-plane smectic C* layer reorientation process is studied for different applied electric field conditions. As expected, the reorientation of smectic layers is strongly slowed down and eventually suppressed by the network, even at rather low monomer concentrations. Polymer network formation in a uniformly reoriented smectic layer state reveals that the network acts in two ways: first it gives a biased elastic torque counteracting a field of such symmetry as to cause a change from the templated layer direction; second it introduces an increased effective viscosity counteracting the reorientation in both directions. The behaviour of samples stabilized by two different kinds of polymer networks, created in between the smectic layers (intra-layer) and across them (inter-layer), is then investigated and discussed.

Rheo-dielectric behavior of low molecular weight liquid crystal 2. Behavior of 8CB in nematic and smectic states

Rheo-dielectric behavior was examined for 4−4′−n-octyl-cyanobiphenyl (8CB) having large dipoles parallel to its principal axis (in the direction of the C≡N bond). In the quiescent state at all temperatures (T) examined, orientational fluctuation of the 8CB molecules was observed as dielectric dispersions at characteristic frequencies ωc>106 s−1. In the isotropic state at high T, no detectable changes of the complex dielectric constant ɛ*(ω) were found under slow flow at shear rates ˙γ≫ωc. In the nematic state at intermediate T, the terminal relaxation intensity of ɛ*(ω) was decreased under such slow flow. In the smectic state at lower T, the flow effect became much less significant. These results were related to the flow-induced changes of the liquid crystalline textures in the nematic and smectic states, and the differences of the rheo-dielectric behavior in these states are discussed in relation to a difference of the symmetry of molecular arrangements in the nematic and smectic textures.

Laser-addressing rewritable optical information storage of (liquid crystalline side chain copolymer/liquid crystals/photo-responsive molecule) ternary composite systems

A laser-addressing rewritable optical information storage for (liquid crystalline side chain copolymer: LCcoP/low molecular weight nematic liquid crystals: nematic LCs/photo-responsive molecule: PM) ternary composite systems in a smectic state was investigated on the basis of a reversible and bistable electro-optical switching driven by ac electric fields with two different frequencies. In order to improve the response speed for the electro-optical switching for the ternary composite system, a LCcoP with the mesogenic side chain fraction of 52.5mol% was synthesized. The ternary composite system with the LCcoP fraction of 36wt% showed a smectic state over a wide range near room temperature. High speed reversible and bistable electro-optical switching (∼100ms) under an appropriate electric field, and a stable memory effect (∼y) were demonstrated upon irradiation of UV light of 355nm by using a YAG laser for the ternary composite systems in a smectic phase at room temperature.

Smectic liquid-crystalline physical gels. Anisotropic self-aggregation of hydrogen-bonded molecules in layered structures

Smectic liquid-crystalline states have been successfully gelled by low molecular weight hydrogen-bonded molecules and anisotropic fibrous aggregates of the gelling agents are obtained when the sol–gel transition occurs within the smectic temperature range.

Fast Bistable Switching Composite Systems of Side-chain Liquid-crystalline Copolymer and Low Molecular Weight Liquid Crystals

Mesomorphic characteristics and bistable light switching properties of the composite system composed of various side chain type liquid crystalline copolymers (LCcoPs) which includes the flexible dimethylsiloxane segment as one constituent of the copolymer and low molecular weight nematic liquid crystals (nematic LCs) have been investigated in order to construct the optimum smectic binary composite system with both fast light switching characteristics and stable memory effects at room temperature. Two types of novel (LCcoP/nematic LCs) composite systems in a smectic phase studied were (1) (pseudo LCcoP with weak polar methoxy terminal groups/nematic LCs with strong polar cyano terminal groups) and (2) (smectic LCcoP with strong polar cyano terminal groups/nematic LCs with the same terminal groups). The response speed of bistable light switching for the binary composite system could be remarkably improved by reducing the LCcoP fraction maintaining a smectic state at room temperature due to an increase of the optimum flexible segment fraction.

Chain-Folded Lamellar Structure in the Smectic H Phase of a Main-Chain Polyester

Smectic state morphology of the main-chain liquid crystal PB-14 polyester was studied by small-angle X-ray scattering (SAXS), differential scanning calorimetry, and transmission electron microscope...

Investigation of the helix unwinding process in thick freely suspended smectic films

Optical and electro-optical measurements have been performed on free-standing chiral smectic films sufficiently thick (10 000 layers) to preserve the natural smectic helix. The Goldstone mode appears at about 200Hz, showing that these films are a much better approximation of the 'ideal' smectic bulk state than a thick planar sample between glass plates (where the Goldstone mode is found at about 3 kHz). In these films the unwinding of the helix is studied, as a function of applied electric field, by monitoring Bragg reflections and their Fourier components. When the helix deforms, a reflection appears which at first sight might be taken for a subharmonic, but must be interpreted as the main 'full pitch mode' reflection relative to the 'half pitch' reflection from the undeformed helix. Our measurements further confirm that in anticlinic materials no helix unwinding takes place prior to the antiferroelectric-ferroelectric transition.

Influence of aging on the mechanical properties of smectic polypropylene

Isotactic polypropylene (iPP) was quenched to the smectic state and analyzed during aging at room temperature for very long times, up to five years. Calorimetric differential scanning calorimetry (DSC) curves, dynamic mechanical behavior, and mechanical properties were used to analyze the aged samples and data were compared with the results for the fresh sample. The obtained results can be explained by a reduction of free volume of the amorphous phase, as well as a concomitant increase of the smectic domains' dimensions. A good correlation was found between some calorimetric and mechanical parameters.

Relationship between Mesogenic Side Chain Fraction in Liquid Crystalline Copolymer and Bistable Electro-Optical Switching for (Liquid Crystalline Copolymer/Liquid Crystals) Composite Systems

The influence of the mesogenic side chain fraction in a side chain liquid crystalline copolymer (LCcoP) on electro-optical effects for (LCcoP/low-molecular-weight nematic liquid crystals (nematic LCs)) composite systems in a smectic state was studied in order to improve reversible and bistable electro-optical switching characteristics driven by ac electric fields with two different frequencies. In the case of the LCcoP fraction lower than 40 wt %, the magnitudes of glass transition temperature (Tg) and the mesophase temperature range for the binary composite systems were independent of the mesogenic side chain fraction in LCcoP, that is, ∼220 K and 220−340 K, respectively. In the case of the LCcoP fraction of 30 wt % and, also, its mesogenic side chain fraction of 52.5 and 32.5 mol %, the binary composite systems showed a homogeneous smectic phase at room temperature. On the other hand, the LCcoP with the mesogenic side chain fraction 16.5 mol % did not exhibit any mesomorphic characteristics at any temperature without any addition of LCs. Then, this LCcoP was named as the pseudo-LCcoP in this study. However, the (pseudo-LCcoP/nematic LCs) composite system showed a novel type of induced smectic phase over a wide range of both mixing concentration (35−85 wt % pseudo-LCcoP) and temperature (220−320 K). This composite system showed a fast switching speed from a transparency state to a light-scattering state. Therefore, it was reasonably concluded that the application of LCcoP with the optimum substituent fraction of mesogenic side chain is useful to realize a high electro-optical switching speed (∼50 ms) as well as a stable memory effect (∼years) at room temperature.